1. Field of the Invention
The present invention relates to a flash device for improving a battery life and suitable for reusing, and a lens-fitted photo film unit incorporating the flash device.
2. Description Related to the Prior Art
Various types of lens-fitted photo film units pre-loaded with unexposed films are sold. There is a type of a lens-fitted photo film unit having a built-in flash device, so that exposures can be taken indoors or in a dark place such as during nighttime. When taking a flash photography by using the lens-fitted photo film unit, a charging switch is turned on by operating a flash operating member prior to photography. Thereby, the flash device is actuated to charge a main capacitor.
The flash device of the lens-fitted photo film unit utilizes a charging circuit which includes an oscillation transformer having a primary and a secondary coils, an oscillation transistor, and a rectification diode as main parts. The oscillation transistor oscillates at a secondary current, which is returned from the secondary coil, to increase/decrease a primary current which flows from the battery to the primary coil. Thereby, high-voltage alternating current is generated through the secondary coil. The secondary current, which flows through the secondary coil, is rectified and output to charge the main capacitor.
The battery is incorporated in a unit body of the lens-fitted photo film unit along with the flash device. The flash device is actuated by using the battery as a power supply. Therefore, the battery is consumed if the user forgets to turn off the charging switch. If the charging switch remains ON for a period of time equivalent to battery life, charging of the flash device cannot be carried out properly. Accordingly, flash emission cannot be carried out. A battery life, for instance, is defined as elapsed time T which is measured as follows. Measurement of elapsed time T starts concurrently with turning on the charging switch for charging. The main capacitor is discharged by emitting flash light once an hour while keeping the charging switch turned on. Charging time T1 is measured every time flash light is emitted. Charging time T1 is between the instant when the flash is emitted and the instant when the main capacitor reaches a predetermined voltage (the voltage which allows the flash emission). Thus, the battery is consumed by continuing charging along with the flash emission. Accordingly, charging time T1 becomes longer. Elapsed time T, which is time elapsed until charging time T1 becomes longer than the predetermined time (for instance, 30 seconds), is defined as the battery life.
Extending the battery life can reduce an influence caused by not turning off the charging switch. For instance, U.S. Pat. No. 6,339,679, the battery life is extended by setting a higher resistance on a path which supplies a base current of the oscillation transistor from the battery. The above reference discloses that the battery life extends as the resistance becomes higher.
A used lens-fitted photo film unit is collected, and the flash device is removed from the used lens-fitted photo film unit. Thereafter, the flash device is reused.
When reusing, it is preferable to increase the resistance of the flash circuit in order to extend the battery life. However, a DC current gain of the oscillation transistor needs to be suitable for the resistance. Therefore, if the resistance is changed, the oscillation transistor also needs to be changed in accordance with the resistance.
It becomes unnecessary to replace the oscillation transistor by setting the resistance within a range suitable for the DC current gain of the oscillation transistor. However, in that case, it becomes necessary to measure the DC current gain and select the resistor having the resistance based on the measured DC current gain. As a result, the reuse cannot be carried out efficiently. It will increase the cost for reusing the flash device.
Further, if there are oscillation transistors with different resistance, it becomes necessary to sort out the oscillation transistor with proper DC current gain based on the resistance. Therefore, it will also increase reusing cost.
The above-mentioned problems can be solved by predetermining the DC current gain so as to use the oscillation transistor irrespective of the resistance. However, it narrows an applicable range of the DC current gain, so that sorting of the oscillation transistor becomes necessary. As a result, it will increase the manufacturing cost.
High-grade camera such as a compact camera or a single-lens reflex camera incorporates a charging control circuit for monitoring the charging voltage to stop charging when charging is completed. The charging control circuit checks the charging voltage at proper intervals or in response to a shutter release operation after completion of charging. When the charging voltage is insufficient, a charging operation is carried out to enable the flash emission. Thereby, unnecessary consumption of the battery is prevented.
However, the above charging control circuit is complicated and expensive. A low cost is one of the characteristics of the lens-fitted photo film unit. Therefore, it is not practical to provide the above circuit in the lens-fitted photo film unit.